A conventional automatic transmission of a vehicle comprises a plurality of clutches and/or brakes, operable in conjunction with one or more epicyclic gear trains to provide a range of speed ratios. Typically a ‘park’ function is provided whereby an output shaft of the transmission is engageable with the gearbox casing to lock the transmission output shaft against relative rotation. The driveline downstream of the transmission is uninterrupted, so that engagement of the park function locks the vehicle wheels against movement.
A conventional driver-operated manual selector for such a transmission typically has the positions PRNDL in sequence, so that the park condition is at one end of those positions. The vehicle ignition switch is usually inhibited unless the transmission is in PARK or NEUTRAL, so that there is no danger of being able to start the vehicle engine with the transmission in gear.
A common means of providing the park lock function comprises a toothed wheel of the output shaft and a pivotable pawl mounted on the gearbox casing and resiliently urged into engagement between adjacent teeth of the toothed wheel. The pawl is disengaged by movement of the manual selector away from the PARK position.
It must be assured that the pawl will engage and disengage from the toothed wheel under all conditions of use. In particular disengagement must be reliable even if the output shaft has torque applied to it from the driven vehicle wheels.
Such a circumstance may occur if the vehicle is stopped on an incline, and the park pawl is engaged to hold the vehicle against movement without application of the handbrake. The park pawl stops the vehicle rolling away, and thus reacts to torque in the driveline.
In another circumstance PARK may be selected before the vehicle has ceased moving, and the park pawl may engage the toothed wheel as the vehicle handbrake is applied. In this case the pawl may trap torque within the driveline.
If torque is trapped or reacted to by the park pawl, it must nevertheless not prevent disengagement of the pawl at a reasonably low disengagement force. A known phenomenon includes rapid ejection of the pawl as a disengagement force is applied, which may be a consequence of the trapped torque. Such ejection is typically accompanied by an audible noise (e.g., “pawl thump”).
Internal measures may be applied to the transmission to solve the problem of such pawl thump, but the phenomenon may not be predictable or repeatable. It may for example be affected by a variable quality of transmission lubrication, slight variation in manufacturing tolerances, or wear and tear.
It would be desirable to find a solution for eliminating pawl thump, which does not require measures to be applied internal to the transmission casing.